Steering is a term applied to a collection of components, linkages, and systems to allow a vehicle to be directed to follow a desired course. The basic aim of steering is to ensure that the wheels are directed to a desired position. This is generally achieved by a series of linkages, rods, pivots, gears, hydraulic systems, etc.
Many vehicles use a rack and pinion steering mechanism where a steering wheel turns the pinion gear and a pinion moves the rack, which is a linear gear that meshes with the pinion, converting circular motion into linear motion along the transverse axis of the vehicle. This motion applies steering torque to ball joints that are connected to tie rods and a short lever arm called the steering gear.
A hydraulic power steering system uses hydraulic pressure supplied by a pump to assist in the motion of turning the steering wheel. There are also electrical power steering systems to assist in the steering of the vehicle.
Four-wheel steering, on some vehicles, is utilized to improve steering response, increase the vehicle stability while maneuvering at high speed and/or to decrease turning radius at low speed. In most active four-wheel steering systems, the rear wheels are steered by a controller and actuators. The rear wheels generally cannot turn as far as the front wheels in most systems. Some vehicles utilize what is called a passive steering system to correct for the rear wheel tendency to toe-out. The passive steering system utilizes lateral forces generated in a turn, through the suspension geometry, to correct the tendency and to steer the rear wheels slightly to the inside of the corner.
Some vehicles utilize rear wheel steering, such as some forklift trucks and some other construction equipment. In many construction vehicles, steering is done by a hydraulic system that is activated by the steering wheel, causing the steered wheels to turn based on hydraulic pressure supplied thereto. Some systems are referred to as steer-by-wire, with the aim of this technology to do away with as many mechanical components, such as a steering shaft, steering column, gear reduction mechanisms, etc. Steering systems that utilize hydraulic power to either assist or to completely control the steering of a vehicle utilize pressurized hydraulic fluid supplied by an engine-driven pump. For many systems, a secondary steering system is also provided to utilize the forward momentum of the vehicle to supply a secondary source of pressurized hydraulic fluid to the system in the event that the primary system ceases to function. For example, if the engine were to cease operation for some reason, steering power is still available as the forward momentum of the machine provides the pressurized hydraulic fluid for the steering system.
The testing of the secondary and primary steering systems can be done in a test stand environment where the systems are activated, deactivated, and checked for functioning using the connections and interaction of the test stand. Another method utilizes a system in which the engine is shut off during operation to see if the secondary system is functioning. This sort of test has several disadvantages in that the engine power is not available and requires the bypassing of safety systems in order to test the secondary system.
What is needed in the art is a secondary steering system test that can be conducted without bypassing safety criteria built into a vehicle's operating system and without the need for a specialized test stand.